We examine models for batch-equilibrium and fractional-equilibrium degassing of CO₂ from magma at Kilauea Volcano. The models are based on

• 1.

  (1) the concept of two-stage degassing of CO₂ from magma supplied to the summit chamber,

• 2.

  (2) C isotope data for CO₂ in eruptive and noneruptive (quiescent) gases from Kilauea and

• 3.

  (3) data for the isotopic fractionation of C between CO₂ and C dissolved in tholeiitic basalt melt.

The results of our study indicate that

• 1.

  (1) both eruptive and noneruptive degassing of CO₂ most closely approach a batch equilibrium process,

• 2.

  (2) the δ¹³C of parental magma supplied to the summit chamber is in the range −4.1 to−3.4‰ and

• 3.

  (3) the δ¹³C of melt after summit chamber degassing is in the range −7 to −8‰, depending upon the depth of equilibration.

We also present δ¹³C data for CO₂ in eruptive gases from the current East Rift Zone eruption. These are the first C isotope data for CO₂ in high-temperature (>900°C) eruptive gases from Kilauea; they have a mean δ¹³C value of −7.82 ± 0.24‰ and are similar to those predicted for the melt after summit chamber degassing. The minor role played by fractional degassing of ascending magma at Kilauea means that exsolved CO₂ tends to remain entrained in and coherent with its host melt during ascent from both mantle source regions and crustal magma reservoirs. This has important implications for magma dynamics at Kilauea.